$rst - 4s + 9t - 1 = 8s - 7t + 10$ Solve for $r$.
Combine constant terms on the right. $rst - 4s + 9t - {1} = 8s - 7t + {10}$ $rst - 4s + 9t = 8s - 7t + {11}$ Combine $t$ terms on the right. $rst - 4s + {9t} = 8s - {7t} + 11$ $rst - 4s = 8s - {16t} + 11$ Combine $s$ terms on the right. $rst - {4s} = {8s} - 16t + 11$ $rst = {12s} - 16t + 11$ Isolate $r$ $r{st} = 12s - 16t + 11$ $r = \dfrac{ 12s - 16t + 11 }{ {st} }$